Static reducing device



D. G. MccAA 2,249,534

sumo REDUCING DEvIE Filed July 6, 1940 July 15, 1941.

' 2 Sheets-Sheet 1 .LNBHBDO BGONV FIG. 2

A TTORNEYS.

Ju|y1s,f1941. D'. G, MCCAA 2,249,534

`sm'rlc REDUCING DEVICE IN V EN TOR.

' DAV/o G. M0044 N BY l www.. 22% E g ATTORNEYS.

Patented July 15, 1941 UNITED YSTATES PATENT OFFICE s'rA'rro nEDr-Jomonnvron David G. McCaa, Lancaster, Pa. Application July, 1940, Serial No. 344,199

1c claims. Erol. 25o-20) This invention relates to electrical communication systems, .and particularly to limitersfor use therein -for the reduction `of high amplitude oscillations, that produce .interfering noises, herein termed static.

The principal object of my invent-ion is to provide means for limiting the energy transferred by a signal transfer circuit, to values of a smaller range than has been attainable vwith the pre` viously known devices intended for this purpose. Other objects of this invention will be apparent from this descriptionl of typical lembodiments thereof, 4and the'various modiiications referred to, and relate particularly to the simplification, refinement, and systematic control of the device of ymy invention. Y

Without limiting the application of the principle of my invention, I will explain it with refer.- ence to drawings of 4simple and useful embodiments, wherein: Y f Fig. l represents one embodiment of my inventiondesigned to operate between the radiant energy collector, such as an antenna -or loop, and receivingapparatusmr continuous oscillations; Fig. 2 vshows the static `characteristic of the herein described embodiments of my invention; variation of grid voltage being plotted against anode current, grid current and anode voltage; Fig. 3 shows the dynamic characteristic of these embodiments of. my invention lwith kgrid Voltage plotted against anode current; v

Fig. 4 represents my invention employedin the rst detector ,in asuperheterodyne receiver; ,and Fig. 5 represents my invention employed in the rst detector in a superheterodyne receiver with I automatic volume control and automatic 'Variation of the grid bias of the ilrst detector..

In Fig. 1,! represents an antenna,"2 aprimary inductance which is grounded Yat 3.l The` sec? ondary inductance 4 is coupledy to the' primary 2. The variable tuning capacity 5 is connected in parallel to the Ainductance 4. The tuned cir; cuit 'comprising inductance 4 and (capacity 5 is connected to the screen grid tube '6 asVV follows: One terminal of the condenser 5 connects to the control lgrid 1. The remaining terminal of the condenser' 5 connects tothe ground at'3. 'Ihe unipotential cathode 8 is actuated by the heater 9, which'may be energizedbya battery vorfby alternating. current. Y The cathode 8: is connected to the ground 3 throughthe biasing resistor Ill; which is by-pas'sed'by the-condenser II." The screen grid I2 connects vto the positive terminal of the battery I3 which isrbyepassed by theconV denser I4; the negative terminal of the'battery I 3 connects to the ground 3. TheV anodeIS connects to one terminal of the primary inductance I6, the other terminal of inductance I6 being connected toV the positive terminal of the'battery IYI, which is lay-passed Vby the condenser I8.- The negative terminal of the battery Il connects tothe positive terminal of the battery I3. Y

While Aa suitable carrier-frequency amplifier has been described above, any. other suitable'device may be used for the purpose of selecting or amplifying the incoming signals 'or for both purposes, or if desired, these functions may be omitted.

The secondary inductance I9 is coupled to the primary I6. The variable tuning capacity 20 is connected in parallel to the inductance' I9 through the large capacity of the by-pass condenser 2I; The tuned circuit comprising inductance I9, and condensers 20 andV 2l Yis-connected to `the screen grid tube 22 as follows: One terminal of the condenser 20 connects tothe con,- trol-grid 23, the remaining terminal of condenser 2l! connects tothe unipotential cathode 24 which is actuated by the heater 2-5 which `may be `energized by a battery or by alternating current. The control grid 23 ishbiased negativelyby adjusting the potentiometer 26, which is shunted across the battery 21. The positive terminal o1' battery -21 and the cathode 24 are both grounded at 28. The by-pass condenser 2| keeps the radio frequency currents from flowing in potentiometer 2li-.and battery 21. The screen grid 29connects to the positivete-rminal of the battery 30 which is by-passed -by the condenser 3l. The negative terminal of battery 30 connects to .the cathode The anode 32 connects to one terminal, of the primaryinductance 33; the remaining terminal of inductance 33 connects to. the con. A denser .34 .and the resistor 35. The -remaining the cathode .24

42.- The ,inductance 33- is tapped at 43 .and the tap connects to the small Variable condenser'M. Theremaining terminal Dicen-denser 44 connects to the -grid V23.-

. At win thus be seen that the Cima of tuner-2 is generally' that of a conventional carrier-fre-Y quency amplier, except that special valuesjof opera-ting unidirectional potentia1f-may conveniently be chosen for the"grid','plate, and scre'en electrodes, Varmi unusual .care is taken: to prevent` the transfer of energy through the vtu"be,^other thanv-through'the electronic path. Upona full Y understanding of my invention, as described The Vnegative ter- V herein, it will be apparent that there is full freedom of choice of the circuit and parts used, so far as it is consistent with the above stated exceptions.

In the oper-ation of the cir-cuit of Fig. 1, the tuned circuits 4, 5 and I9, 20 and the tube 6 comprise a radio frequency amplier and frequency selector.- The reduction of high amplitude oscillations takes place in the tube 22 and is the result of the arrangement and design of the anode or output circuit 33, 34, 35 and 36, in cooperation with the proper values of bias on grid 23.

Referring to Fig. 2, the static characteristic of .the tube 22 and its associated circuits is represented by the graphs and can be produced by using any suitable triode or pentode, selecting the proper resistance value for resistor 35 and adjusting the impressed voltages taken off the potentiometers v36 and 26. With type 51 or 11 pentodes and with type 21 or 56 triodes, I have usedvalues varying from one-half to ten megohms for resistor 35 with impressed voltages from cathode 24 to the potentiometer slider 36 varying from zero to seventy-five volts. `More specifically, inV a satisfactory embodiment using a type 51 tube, resistor 35 was five megohms with impressed voltage taken 01T the potentiometer 36 varying from zero toforty volts, twenty volts being a suitable value. The maximum negative grid bias on the potentiometer 26 may be six volts. The screen voltage battery 30 is Vmade low in voltageranging from three to ten volts, three volts having been successfully used in this embodiment.

The general shape of the static characteristic represented in Fig. 2 is described in my United States Patents Nos. 2,095,261 and 2,112,705. With the grid voltage at zero, the anode current is substantially the same as if 'the anode to cathode path were lreplaced by a low resistance, the anode current being largely determined by the resistor 35 and the impressed voltage at the slider 36. Grid current appears when the grid voltage is zero and for positive grid voltages it rapidly increases, while the anode current rem-ains substantially constant. As the grid voltage is adjusted from positive to negative, the grid current falls, reaching substantially zero at the ordinate 45; while the Ianode current remains constant until the grid voltage is at the more negative value at ordinate 46; the anode current then decreases steadily as the grid is made more yand more negative, until finally zero anode current appears at rthe value of negative grid voltage indicated by the ordinate 41.

The anode to cathode voltage is very low at zero or positive grid voltages, owing to the large voltage drop across the resistor 35 and it re mains low as the grid voltage is adjusted toward the negative until ordinate 46 is reached, then as the grid is made more and more negative, the anode voltagersteadily rises because, as the anode current in resistor v'35 decreases, the voltage drop across the resistor decreases. Finally at ordinate 41 Value of negative gridA voltage, the anode current is zero and the anode voltage is the same as the voltage at the source of voltage on potentiometer 36. i

The dynamic -characteristic ofthe anode circuit, as` used in the :above mentioned patents, is closely 'similar to the static characteristic de'- scribed above. -At a selected operating point on the sloping part of the anode current curve, when the signal voltages make the grid more soY negative the anode voltage increases, and for less negative grid voltages the anode voltage decreases. These changes in the anode voltage caused the anode current changes to be small and the output had to be taken 01T the anode resistor as voltage.

As will be clear from a study of the circuits of my above mentoined patents, the operation of the described devices is dependent upon the impedance between the anode and the cathode but external .to the electron stream, being substantially the same at the working frequency as the resistance. The in'ternal capacity of the tubes generally available, between the cathode and the anode, was entirely detrimental in its effect, and was one factor which limited the effectiveness of the method. In lthe present invention, however, the above mentioned limitation is negligible, and the operation of my present invention depends upon such different factors that I have included condenser 34 as an important element of the described embodiment.

My present invention thus differs widely from those of the above mentioned patents in the use of the condenser 34 land the output primary coil 33 in the anode circuit. Condenser 34 is made one-tenth microf'arad or larger and the primary 33 is made of a usual value. The anode circuit of the tube 22 in Fig. 1 presents the static char- -a-cteristic shown in Fig. 2. The dynamic characteristic is, however, widely different as represented in Fig. 3. The condenser 34 operates as a storage reservoir for electrical energy, the quantity of ener-gy stored varying with the size of the condenser and the voltage impressed upon it. The voltage impressed upon the condenser 34 is substantially the same as the anode voltage in Fig. 2, therefore the charge in condenser 34 is Very small when the grid voltage iszero or positive vand it remains small as the grid voltage is adjusted toward the negative until the ordinate 46 value of negative grid voltage is reached; then, as the grid is made more and more negative, the anode current falls, the anode voltage rises, and the charge in condenser 34 steadily increases until the ordinate 41Y value of negative grid voltage is reached, when the charge is at .a maximum value determined by thevoltage atthe source of voltageon potentiometer 36.

Since a charged condenser operates momentarily like any other source of voltage, such as a battery, the graphs in Fig. 3 were-obtained by substituting a battery voltage for the system 34, 35, 36, and adjusting the voltage so as to produce the same anode current as found at'the selected operating points (ordinates 48, 49 and 50) when using the system 34, 35 and 36. With the same anode current in the two cases, the same anode voltage is present, the grid voitage remaining the same in each case. Fig. 3 shows the static characteristics of Fig. 2 with selected operating grid voltage points 48, 49 and 50 with the system 34, 35 and 36 in use. Substituting a battery for 34, 35 and-36 yand adjusting the voltage for the same anode current Yat the same grid voltage or operating point, the grid vol-tage was increased and decreased in smallsteps above and below the selected operating point and the resultant anode current changes in each case are represented by the curves 5|, 52 and 53.

Plotted curves show that the static characteristic curve is effectively a rectilinear (odd order) type, while the dynamic curves follow the even order or square law type.V

If condenser 34" is charged to the maximum value by increasing the negative grid voltage to` a value greater than required for zero anode current and the connection to resistor 35 is then opened, a xed charge remainsv in the condenser 34. Under this condition, if signal voltages are impressed upon the grid and the unidirectional grid voltage is slowly reduced until each positive wave of the signal causes anode current to ow in the tube 22, there is -a momentary signal output in the circuit 38, 39. The energy stored in the condenser 34 is soon exhausted as each positive wave of signal causes a unidirectional current to flow in the primary 33 and tube 22 each time the tube impedance is lowered. The time required to exhaust theV charge in condenser 34 varies with the signal amplitude. The quantity of energy exhausted per unit of time represents a denite rate of ow of energy out of the condenser 34 which is dependent upon the amplitude of thev signal oscillating voltages on the grid. This constant drainage also appears when the direct grid voltage is set to the operating point required by the signal amplitude.

Another action in the anode circuit of Fig. l is found inthe time constant presented by the condenser 34 and the resistor 35. The rate of flow of energy or the quantity of energy per unit vof time delivered to the condenser 34 is dependent upon the value of the resistor 35, the impressed voltage at the voltage source 36, and the voltage drop across the resistor caused by the anode current. Owing to the square law character of the dynamic characteristic, signal oscillations cause the aver-age anode current to rise no matter where the operating point is selected along the static characteristic. This rise of anode current decreases the rate of flow of energy into condenser 34 since the voltage drop across the resistor 35 is increased and the impressed voltage is thereby decreased.

The rate of flow of energy into the condenser 34has been controlled by other means. Without using the resistor 35, intermittent contacts can be made between the condenser 34 and the source of voltage 36; the quantity of energy delivered per unit of time then varies with the number of contacts per unit of time, the time duration of each contact and the voltage at the source 36. A reactance coil or a resistor will retard the rate of flow for any combination of the last above factors, if placedy between the voltage source 36 and the intermittent contact device.

The manual operation of the system shown in Fig. `l is 'as follows: with a radio receiver connected to post 4I and tuned to a desired signal frequency the circuits, 4, 5; I9, 20; and 38, 39 are tuned to the same desired signal frequency. With the tubes 6 and 22 in operation, the voltage source 36 is set toa given value, for example twenty volts, and the grid vol-tage is adjusted on the potentiometer 26. With the signal and higher amplitude static oscillations induced in the circuit 4, 5, they are amplifiedV bythe tube 6 and delivered to the circuit I9, 29. The adjustment of the grid voltage at the potentiometer 26 will canse signal to appear as the unidirectional negativeY voltage is slowly increased and the static 4.oscillations will be markedly reduced, pro-l vided the grid voltage is not made more negativeY atpotentiometer 26 than enough to transfer a satisfactory signal, because the rise in the average anode current due to thesquare law dynamic characteristic opposes the fall in anode current due to the direct gridV voltage at potentiometer 26 and causes just suicient energy to be delivered to condenser 34to accommodate the desired signal by supplying energy at a rate equal to its consumption by the tube 22- as actuated by the signal oscillations. High amplitude ,static'oscillations are not accommodated under this condi'-v tion and by exhausting the chargeony condenser 34 they appear to cyclically change the operating characteristic. potentiometer 26 or the impressed voltage at the source 36 are increased beyond the proper values Y for any given signalamplitude,A the staticoscillations begin toy increase inthe output circuit 38, 39 since both of. these adjustments increase the rate of now of energy into the condenser 34 to a value thatis greater than the `rate of floWf out of the condenser 34ycaused yby the signal.

In general the source of voltage 36 can be left xed and a-large range of signal valuescan be accommodated by varying only'the directi grid y voltage at the potentiometer 26; weak signals will requireless and stronger signals more negative voltage at 26. Y j

With a sensitive radio receiver connectedV to the circuits of Fig. 1, signal and 4static oscillations will be transferred from the circuit I9, 20 to the circuit 38, 39 by theY small capacityof the grid to the anode in a screen grid tube and may l cause undesirable eiects. The variable condenser 44 isV therefore adiustedto neutralize this effect; the windings 33 and 38 having been given the proper sense of directionfor such purpose. A

Two actions, repeatedly found, coniirm` the above described actions: Y

First: With the system properly adjusted, if the antenna is removed, each time it is put on there is a loud momentary sign-al and if static oscillations are present, they arenot greatlyV reduced during a momentary period; and

Second: With the system 'properly adjusted, as the circuits 4, 5 and I9, 20 are detuned slightly, strong static oscillations appear on either side of the tuned or resonant adjustment.

Both of these actions aredue. to the increase of the rate ofow of energyi-nto condenser'3'li when the signal carrier is absent,.since the fall of anode current due to'- the direct grid voltage at 26is not opposed by the rise of the average anode current kdue to the square law dynamic characteristic and thevoltage impressed upon condenser 34 ishigh.v

In Fig. 4, 54 represents an antenna, 55 aprimaryinductance'which is grounded at 56.` 'I'he secondary inductance 5`Iis coupled to the .primary 55. One terminal of the inductance 5'I.connects to one terminal of the inductance 58'; the remaining terminal of the inductance 58 connects to the ground 56. I'he remaining terminal of inductance 51 connects tothe variable tuning capacity 59; the otherV terminal ofthe condenser 59 vconnects to thek ground' 56. vThe inductance Ellis coupled to the inductance 58 vand is supplied with oscillating radio frequency energy' from the generator 6I.' 1

The tunedcircuit, comprising inductances 51, 58 and the condenser 59, is connected to the elec-4 tron tube 62 vby joining the'control grid 63 tothe commonr terminals of the inductance'51and the condenserV 59.Y The Qunipotential" cathode 64',

which is actuated'by' the heater 65 vvvh'iclri maybe energized by a'battery or by alternating. current,-l isv connectedA to the slide on the potentiometer Yist whichisbypassedftoithe groundssby-tne con;

denser s1. The potentiometer as `isf shun'tec Ifithe grid voltage at' across the battery 68; the negative terminal of battery 68 connects to the ground 55.

The anode 69 connects to one terminal of the primary inductance 10, the remaining terminal of the inductance connects to the condenser 1I and to the resistor 12; the remaining terminal of the condenser 1I connects to the ground 56 and the remaining terminal of resistor 12 connects to the slide on the potentiometer 13 which is shunted across the'battery 14; the negative terminal of the battery 14 connects to the ground 56.

The secondary inductance 15 is coupled to the primary 10 `and the variable tuning capacity 18 is connected in shunt to the inductance 15. The circuit comprising inductance 'I5 and the condenser 16 is tuned to the intermediate frequency and connects to the intermediate frequencyamplier 'I1, which connects with the second detector 18, audio amplifier 19 and speakerBO. v

In the operation of the circuit of Fig. 4, the circuit comprising 51, 58 and 59 is tuned to the desired signal frequency, the generator 6I is adjusted to produce a radio frequency which, by detector action, Will produce the intermediate frequency to which the circuit 15, 16 is tuned. The source of energy 13 is set to 'a preselected value and the slide on potentiometer 66 is gradually movedroff the ground toward the positive end until the signal appears. The range of signal that will be detected, with marked noise reduction, will vary with the value of the biasingr voltage at potentiometer 66 and the voltage at the source of energy 13. Strong signals will require higher biasing voltage and morefvoltage at the source 13 and weak signalsvvill require less voltage at potentiometers 66 land 13. The even order or square lawVV dynamic characteristic is particularly favorable for detector action and the detector presents Well defined limitation of transient oscillations Without 'distortion of the resultant desired audio signal.`

In general, the usual values Aof the parts useful in the first detector of a superheterodyne receiver will serve the purposes of the device of Fig. 4, except that the resistor 12, the condenser 1| and the electrode biases should be the vsame as those used in the device of Fig. 1. The neutralizing condenser of Fig. 1 is not essential inV this case, as the frequencies to which the inputand output circuits of tube 62 respond are so different that the transfer by the tube capacity may generally be ignored, but of course the tube 62' may be neutralized if desired.

In Fig. 5, 8| represents an antenna, 82 a primary inductance which is groundedv at 83. The secondary inductance 84 is coupled to the primary 82. The variable'tuning capacity 85 is connected in parallel to the inductance 84 through the large capacity of the by-pass condenser 86. The circuit comprising 84, 85, 86 is connected to the screen grid tube 81 as follows: one terminal of condenser 85 connects to the control grid 88, the remaining terminal of condenser85 connects to the ground 83. The unipotential cathode 89, which is actuated by the heater 90, whichv may be energized by a battery or by alternating current, connects to the'ground 83 through the biasing resistor 9| which is'shunted by theby-'pass condenser 92 and provided with a variable sliding contact 93. The screen grid '94 connects tothe positive terminal of the battery 95l Whichis by-passed by condenser 96; the negative terminal of battery 95 connects to the ground 83. Theanode 91 connects to'one terminal of theprimaryinductance 98, the other terminal of inductance 98 being connected tothe positive terminal of the battery 99 which is by-passed by the condenser |00; the negative terminal of battery 99 connects to the positive terminal of battery 95. Y

Thesecondary inductance |0| is coupled to the primary 98. One terminal of inductance |0| connects to one terminal of the inductance |02, the other terminal of inductance I02 connects to the slide 93 on resistor 9| which is by-passed to the ground 83 by the large condenser |03; the remaining terminal of inductance |0| connects to the variable tuning capacity |04; the other terminal of condenser |04 connects to the ground 83. The inductance |05 is coupled to the inductance |02 and is supplied with oscillating radio frequency current from'the generator |06. I

The tuned circuit comprising IOI, |02, |03, |04

is connected to the electron tube |01 by joining the control grid |08 to the common terminals of inductance |0| and condenser |04. The unipotential cathode |09, which is actuated by the heater IIO which may be energized by a battery or by alternating current, is connected to the slide on the potentiometer III, which is by-passed to ground 83 by the condenser |I12. Potentiometer III is shunted across the battery H3; the negative terminal of battery ||3 connects to the ground 83. The anode yI III connects to one terminal of the primary inductance I5; the variable tuning capacity I I6 is connected inshunt to inductance II 5, the remaining'terminal of inductance II5 connects to the condenser II1 and to the resistor |I8; the remaining terminal of condenser |I'I connects to the ground 83 and the remaining terminal of resistor II8 connects to the slide on potentiometer I I9 which is shunted across the ybattery |20; the negative terminal of battery |20 connects to the ground 83.

The Secondary inductance |2| is coupled to the primary I I 5 and the variable tuning capacity |22 is connected in shunt to the inductance I2I The circuits I I5, I'I6 and |2I., |22 are adjusted to be tuned to the intermediate frequency and circuit |21, '|22 connectsftothe intermediate frequency ampliiier |23 which connects With the second detector |24', audio amplifier |25 and speaker |26. A voltage of constant polarity is obtained in the second vdetector by rectification of the signal oscillations; this voltage is transferred by the connectingwire |21, through the inductance 84 to the control grid 88' in the tube 81.

"The operation of the, tuned radio frequency amplier, comprising circuits 84, 85, 86 and IOI, |02, |03, |04 with the tube 81; the action of the local generator |06 and the production of the intermediate frequency by the action of the rst detector, are Wellunderstood in the art. In this embodiment, my invention resides in the combination of these parts with the circuits ofV a first detector constructed to' use the operatingcharacteristics describedv in'connection with Figs. 2, 3 and l and controlled in the range of signal it will fully accommodate by a voltage derived from the signal oscillations. A

As describedv in connectionv with Fig. 4, the range ofthe` detector increases and decreases with the value ofthe direct or biasing voltage on 'the grid. To make the signal oscillations supply a propermbiasing voltage the cathode |09 -is made positive to the ground by the voltage along thepotentiometer |'II and when the slide 93 is moved from the ground toward the positive end of the cathode resistor 9|, the grid |08 is made'positive to the ground; this requires a further increase of the voltage along potenhave found that the following 25249;534 tiometerl l in order to=obtain anygivenoperating point. vThe j--twowoltage sources -93 and |||-can be adjusted so that the firstdetectoris adjusted toV accommodate only a Avery'smalksigvoltage on the grid 88 decreasesthe anodeand vscreen currents `in the tube"81 kand therebyreduces the amplification; so the first detector circuit does not require'as'wide a range since the signals applied to it'approachV a Lconstantlevel or amplitude. The voltage across Aresistor 9| is established by the anode and screen "currents in the tube 81 and as 'these currents "arereduce'd Ythe grid |08'becomes less positive toitheground yand more negative to the cathode Y|09. The

range-ofthe detector |01 is thus increa'seduntil it fully` accommodates the signal and if the'sigvnal weakens orfades, the range of the 'detector decreases. The `variation in Vrangeof the'detector is partly dependent upon the value ofthe source of voltage at ||9 since the energy output Vis greater at the higher voltagevalue's. 'The 'de- `scribedkindof automatic control is alsooperative ifmy invention is Vused in an amplierinstead of a detector. y

VThe same Values-at'l'l1, ||8,"|20'an`d at IH, H3 can be used as described in connection with Fig. 1.

In capitulationgwhileit is clearfrorn the description of my inventionthat"embodiments "of it'are not restricted toany/partic'ular Values, I

values are suitable for the Vdescribed embodiments.

, In Figure V1: Tube Y22 Type 57 y Source 26 One to. two volts Source 21 Six volts Source 30 Three volts `Condenser 34 One-tenth microfarad Resistor 35 Five megohms Source 36 Twenty `volts Source 31 .Forty volts In Figure 4: Tube 62 Type 56 y Source 66 One to two volts Condenser 01 One-tenth microfarad Source' 68 Six volts Condenser 1| One-tenth microfarad Resistor 12 Three megohms Source 13 Twentyvolts Source 14 Forty volts In Figure Y Tube 81 Type 58 Resistor 9| One thousand ohms Condenser 92 One-tenth microfarad Source 93 One-half to one volt Source 95 One hundred Volts Source 99 One hundred and fifty volts Tube |01 Type 56 Source One to three volts Condenser ||2 One-tenth microfarad Source 3 Six volts Condenser ||1 One-tenth microfarad Resistor IIB Three megohms Source H9 Thirty volts Source *Fiftylvolts While lI fdo;v not Y wish to ,be 'restricted --to-' 'any Vtheoryof operation, none of the numeroustests` Vwhich-"I have made is inconsistent theory outlined below. e

In the Voperating condition, with thenormal Lanode and Vgrid voltagessupplied,` but with `*no s'ignalfpresent, I the effective anode4 impedance to a highfrequency vsignal is very high compared to the load impedance vformed 'by the output primary and Y the large condenser, Aand the -anode -current has predominatinglyan even-order rela- "tion to a small alternating-.grid 4voltage-astypilied-hy the second poweror square law characteristic. Under this condition, upon application 'f5 of' increasing alternatinggrid voltages, the unidirectional -anode current tends to increase. VVThis tendency of the average anode current to 'increase results-=in-achange ofthe anode'voltage to a less positive or toward'thenegative tgoevalue, due to the presence of the Vresistor-and `thedropl in voltage across it, and the tube 'will uthen' operate as if-'a less positivedirect Apotential nwerebeing applied'across the terminals voli the condenser. Y The tendency of the oondenserto 25 maintain `Aa constantvoltage vprevents theaforesaid change of anode potentialfrom occurring as-rapidly as it otherwise would but' duetoA the nite `Value of the condenser charge, the change v. appears'in a brief `f time andA the greater 'the 30 valuebf lthe applied -alternating'grid Voltage,

the more' rapidly the change `takes place.

'It ischaracteristic of the normalfelectronA tube, 'used as describedhere-in and considering onlya 'xed steady-grid voltage, that, corresponding to "the above mentioned Wregion where 'a predominatingly even' order relation'exists between' vthe grid voltage and the anodev current, there'is a region corresponding rto `lower anode 'yoltages, 'where the even order Arelation becomes fles's marked andv where the odd'orderrelati'on tends-to predominate, as is 'typified byV rectilinear operation with third harmonic distortion. `In 'the latter region, a change Vin the 'alternating .grid voltage does not tend' to vmake a substantial '.15 change inthe average anode current-and in fact, beyonda'certain limit increases in alternating grid Avoltage cause substantially no change in the alternating anode currentV of the tube. Y It appears to be also characteristic ofelectron VVtubes, when operated as described herein, that /asthe elective anode voltage becomes less .posi- Vtive,'the amplification factor of the tube becomes less. AAccording to this theory, therefore, assumingthat an operating point has been chosen where, for the Vestablished anode supply voltage, a change of the direct gridvoltageftoward the positive would increase the anode current, if an alternating'Voltage` is applied to the grid, the amplification `will be reduced as the signal is increased 'and there'is a 'limit of the possible output current available 'from the tube. VAThe 'decrease 'in amplicationvand the approach to the limitingcondition will occur rapidly when a relatively. large signal is applied to the grid. Following this theory, a signal, which is small `compared with thatfrequired to establish the klast mentioned condition, willbe amplified .to a 'reiativeiygreat extent, whim a Signat which is of a large order, will be amplified Yto a much smaller extent, and, to a certain extent, limited in the' possible response which-it may cause.

'Due to the diiiculties Vof measurement, Ido not Want to designate either the reduction of amplik7. cation or the output limiting effect as the one "whichis more 'effective 4in devices utilizing my invention, and it appears that both effects are useful, the predominating utility of one over the vother depending on the voltages applied to the apparatus. Y

Applying this theory to the specific case of the reduction of the relative strength of static interference in the presence of signal, the application of vthe signal causes an increase: of the average anode current, substantially up to the amplification. The static, however, cannot cause any substantial increase in the average anode current, the limit having been reached, sorits Vtendency to increase the average anode current Aresults in reducing the effective direct anode voltage morerperapplied alternating grid voltage than the signal does, and the resulting effective direct current anode operating condition'is one where the amplification is relatively small and output limitation tends to occur. It is evident that 'the signal will be ampliiied to a much greater extent than the static, and that the static will tend to be limited.

` useful.

to reduce Ithe-peak amplitudes ofv signals of the type of static, relative. tok the peak amplitudes of the accornpan'ying signal in the same circuit. Whileextending the time during which .the oscillation due to theA disturbing signal has peaks of amplitude comparable withthe peaks of the signal;v and second, that .no knownvarnplitude limiter will suiciently change its response characteristic over s'uch a small range of stimulus that'it will limit the amplitude of the static signals to nearly the peak amplitude of the defsiredsignals when lthe amplitudes are so small that' radical reduction would be commercially Theoretically, Ythese difficulties can be overcome, by amplifying the signals in the absence of selective circuits, but vpractical considerations have prevented the commercial adaption of such'structures.

The theoretical distinction between the amplitude limiter and the energy limiter is not in the circuit, or in the interconnection of parts, as the apparatuses resemble each other in this respect,

A most important result of the change from even order operation to odd order operation as the applied alternating grid voltage is increased is that if, instead of'a signal of the original applied frequency, ther-e is chosen a derived signal depending only upon the even order components of the output signal, the amplitude discriminating properties of devices of my invention are enhanced. As a particular illustration, when a circuit according to my invention is used as a detector, as described in Fig. 4 and Fig. 5, the selected output is very effectively increased in arnplitude with respect to the applied interfering static. This is true of the so-called first detector of a superheterodyne receiver as well as the second detector.

In order to avoid confusion with the known devices which have for their purpose the limitation of the amplitude of the output signal available from them, which will be referred to as amplitude limiters, the devices of the present invention will be referred to as energy limiters. The practical distinction is that, when received, radio signals in which static is present to such an extent that disturbing noise would be apparent at the output of the radio system, are applied to the input through my energy limiter, the resulting degree of reduction of the disturbing noise in the output is not approached by any of the many apparatuses which I have tested which involve an amplitude limiting device inserted anywhere, before, in or after, the usual radio system.

Amplitude limiters are characterized in that they change in operating characteristic from full-response to no-response upon a change instimulus. Many wherein the required stimulus vfor complete change is a small percentage of the range of stimulus for which full-response occurs, are known in the art, but their applicabilityto the field of reduction of static in the prespeak'values of the desired signals in' the input circuit by many times, the well known effect of the selective circuits traversed by the signals is but principally in the relative values of the electrical parts, which provide an embodiment of the principle. The difference may be understood from the knowledge that the operating characteristic of the amplitude limiter must change radically at the end of the operating range,

Vwhereas, in the energy limiter, only sufficient output energy is available to respond to the stimulus of the desired signal, and for values of stimulus beyond the range of the desired signal, no energy remains to convey a signal to the response system.

The description in the specication is sufficient to enable one skilled in the art, to build and to operate a device according to the principle of my invention, but it is illustrative, and not definitive.

My invention may be applied in any instance Where it is desired to limit the response energy regardless of the extent of stimulus, by limiting the energy available for response purposes at that instant. The following arrangements are cited as devices using the principle of my invention:

(a) A device wherein the limited response energy is alternating or pulsating, rather than continuous as illustrated in the detail herein. The alternating energyfmay be synchronous with the stimulating forces, and in any operating phase relation therewith, being derived. for example, from the incoming signal after limitation; or it may be asynchronousin which case usev may be made of response signals having a new frequency bearing a simple Anumerical relation to two applied irequencies. Such devices may be termed sources of response energy.

(b) Sources of response energy'supply, limited in themselves such,'for example, as photo-electric sources, may be used to supply the limited energy required. Various means other than those described may be used Ato limitrthe energy supplied to the response device. A typical example is a vacuum tube operating at saturation current. These arrangements may `be referred to as energy limiting means.

(c) Various means may be used to store the limited quantity of response energy, of which an inductor associated with a magnetic field is a typical example. These arrangements are termed energy storage means. A

(d) Various arrangements for adapting the range of operating capability of the device to the operating range of the desired signal may be used. The value-.of the signal maybe modified -to suit the` device, or theV range of thedevice may bef changed to accomodate the applied signal,

or both'may be changed.. These arrangements are termed control means.

(e) The control means for changing the operating range of the device may be-operated. in

accordance with the -incoming signal, either belimit. These'arrangements are termed automatic f or limited control means.

(f) Whenever itis desired to use an amplitude limiting device in conjunction with an energy limiting system or incorporated therein, any device, which performs the'functions referred to in the foregoing discussion of such devices, may be used. Y

In every case where a device'is used which Cliffers .substantially from the devices described herein in detail, it is understood that it will be a usefulembodiment of myinvention only to the extent that its capabilities are effective over the range of stimulus applied to it in operation.

What I claim is:

1. Anelectrical signal transfer system fordiscriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations, comprising a single vacuum tube having a cathode, acontrol grid, and an anode; an input circuit connected to said cathode and said grid; an output circuit connecting said cathode and said anode; said vacuum tube having a curved relation between the voltage between said grid and said cathode, and the current flowing between said cathode and said anode, said output circuit including an energy storage device and an output impedance in series relation therewith; and a source of limited response energy for said output circuit including an energy limiting element, said element being connected in a shunt path with respect to said energy storage device in said output circuit; the effective impedance of said source being substantially greater than the impedance between said anode and said cathode under the operating conditions and at least a thousand times as great as that of said device at the operating frequency.

2. An electrical signal transfer system for discriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations, comprising a vacuum tube detector having a cathode, a control grid, and an anode; an input circuit connected to said cathode and said grid; an output circuit connecting said cathode and said anode; said vacuum tube having a curved relation between the voltage between said grid and said cathode, and the current flowing between said cathode and said anode, said output circuit including an energy storage device and an output impedance in series relation therewith; and a source of limited response energy for said output circuit including an energy limiting element, said element being connected in a shunt path with respect to said energy storage device in said output circuit; the effective greater" than the: impedance between said anode and said cathode under theoperating conditions `and at `least a thousand'times as great asthat of said device atthe operating frequency,

3. An electrical signal transfer system for discriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations,.comprising a single vacuum tube having a cathode, a control'grid vand an anode; an input .circuit connected to said cathode .land said grid; meansmaintaininga difference of. effective unidirectional potential between the' surface of said gridand said'cathode; an outputl circuit connecting said cathode and said anode; said vacuum tube having'a curved relal Ation between the voltage between said grid and said cathode, andthe current flowing between said cathode Aand said. anode, said output circuit including .an energy storage device and an outnk put impedance in series Vrelation therewith; yand asource of limited response energy for said output circuitfincludingfan energy. limiting element, said element vbeing connected in .a'shunt path with'respect to said energy storage device in said output circuit; the effective impedance of said source being substantially greater than the impedancebetween said .anode Vand said cathode under the .operating conditions and at `least a thousand times as great as that of said device .at the operatingfrequency. f

4. An electrical signal transfer system comprising `atransfer device having a first element, a control element, and .an output element; aninput circuit including said icontrol element, a sourceuof unidirectionalfenergy for said control element, and an output circuit including said output element,i.one of said circuits including said rst element; :said transfer device4 having a curved relation between the stimulusrinfsaid linput circuitand the 'response in said output'cir'- cuitsaid output circuit including an energy storage device andan outputlimpedance in series relation therewith; and a source of limited response energy for said output circuit including tially greater than the impedance between said n' anode and said cathode under the operating conditions and at least a thousand times as great as that of said device at the operating frequency.

5. In a radio receiver, a demodulator circuit containing an electron tube; an anode circuit for said tube, an output impedance and storage means for electrical energy, for supplying response energy, in series relation in said anode circuit; a source of energy; and means for supplying said storage means with electrical energy at a limited rate from said source.

6. In a superheterodyne radio receiver, a first detector circuit containing an electron tube; an anode circuit for said tube, an output impedance and storage means for electrical energy, for supplying response energy, in series relation in said anode circuit; a source of energy; and means for supplying said storage means with electrical energy at a limited rate from said source.

7. In a superheterodyne radio receiver, a rst detector circuit containing an electron tube; a grid circuit for said tube, and an anode circuit for said tube; said Ygrid circuit being supplied with a unidirectional voltage varied by the signal oscillations applied to said receiver, an out# impedance of ,said source :being substantially put impedance and storage means for electrical energy, for supplying response energy, in series relation in said anode circuit; a source of energy; and means for supplying said storage means with electrical energy at a limited rate from said source.

8. An electrical signal transfer system for discriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations, comprising a single Vacuum tube having a cathode, a control grid, and an anode; an input circuit connected to said cathode and said grid; an output circuit connecting said cathode and said anode; said vacuum tube having a curved relation between the voltage between said grid and said cathode, and the current flowing between said cathode and said anode; a primary source of electrical energy for said output circuit; a secondary storage source for supplying electrical energy to said output circuit; means, having an impedance substantially greater than the impedance between said anode and said cathode under the operating conditions, for supplying said secondary source with electrical energy from said primary source at a limited rate, and means for supplying energy from said secondary source to said output circuit whereby the energy in said output circuit is available to produce a substantial response for the desired low amplitude oscillations and a limited response for the undesired high amplitude oscillations.

9. An electrical signal transfer system for discriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations, comprising a vacuum tube detector having a cathode, a control grid and an anode; an input circuit connected to said cathode and said grid; an output circuit connecting said cathode and said anode; said vacuum tube detector having a curved relation between the voltage between said grid and said cathode, and the current iiowing between said cathode and said anode; a primary source of electrical energy for said output circuit; a secondary storage'source for supplying electrical energy to said output circuit; means, having an impedance substantially greater than the impedance between said anode and said cathode under the operating conditions, for supplying said secondary source with electrical energy from said primary source at a limited rate, and means for supplying energy from said secondary source to said output circuit, whereby the energy in said output circuit is available to produce a substantial response for the desired low amplitude oscillations and a limited response for the undesired high amplitude oscillations.

10. An electrical signal transfer system for discriminating in favor of low amplitude desired oscillations and against higher amplitude undesired oscillations, comprising a single vacuum tube having a cathode, a control grid and an anode; an input circuit connected to said cathode and said grid; means maintaining a difference of eective unidirectional potential between the surface of said grid an-d said cathode; an output circuit connecting said cathode and said anode; said vacuum tube having a curved relation between the voltage between said grid and said cathode and the current owing between said cathode and said anode; a primary source of electrical energy for said output circuit; a secondary storage source for supplying electrical energy to said output circuit; means, having an impedance substantially greater than the impedance between said anode and said cathode under the operating conditions, for supplying said secondary source with electrical energy from said primary source at a limited rate, and means for supplying energy from said secondary source to said output circuit, whereby the energy in said output circuit is available to produce a substantial response for the desired low amplitude oscillations and a limited response for the undesired high amplitude oscillations.

DAVID G. MCCAA. 

